Pumping of the human heart is caused by precisely timed cycles of compartmental contractions of the heart muscle which lead to an efficient movement of blood into the heart and out to the various bodily organs and back again to the heart. These precisely timed cycles are controlled and directed by electrical signals that are conducted through the cardiac tissue and can be referred to as pacing signals.
The sinoatrial node (SA node) is the heart's natural pacemaker, located in the upper wall of the right atrium. The SA node spontaneously contracts and generates nerve impulses that travel throughout the heart wall causing both the left and right atriums to sequentially contract according to a normal rhythm for pumping of the heart. These electrical impulses continue to the atrioventricular node (AV node) and down a group of specialized fibers called the His-Purkinje system to the ventricles. This electrical pathway must be exactly followed for proper functioning of the heart.
When the normal sequence of electrical impulses changes or is disrupted, the heart rhythm often becomes abnormal. This condition is generally referred to as an arrhythmia and can take the form of such arrhythmias as tachycardias (abnormally fast heart rate), bradycardias (abnormally slow heart rate) and fibrillations (irregular heart beats).
Of these abnormal heart rhythms, fibrillations, and particularly atrial fibrillations, are gaining more and more attention by clinicians and health workers. Atrial fibrillation develops when a disturbance in the electrical signals causes the two upper atrial chambers of the heart to quiver instead of pump properly. When this happens, the heart is unable to discharge all of the blood from the heart's chambers thus creating a situation where the blood may begin to pool and even clot inside the atrium. Such clotting can be very serious insofar as the clot can break away from the atrial chamber and block an artery in the brain, and thereby cause a stroke in the individual.
A variety of treatments have been developed over the years to treat atrial fibrillation, namely, treatments to either mitigate or eliminate electrical conduction pathways that lead to the arrhythmia. Those treatments include medication, electrical stimulation, surgical procedures and ablation techniques. In this regard, typical pharmacological treatments have been previously disclosed in U.S. Pat. No. 4,673,563 to Berne et al.; U.S. Pat. No. 4,569,801 to Molloy et al.; and also by Hindricks, et al. in “Current Management of Arrhythmias” (1991), the contents of which are herein incorporated by reference.
Surgical procedures, such as the “maze procedure”, have also been proposed as alternative treatment methods. The “maze” procedure attempts to relieve atrial arrhythmia by restoring effective atrial systole and sinus node control through a series of incisions.
The maze procedure is an open heart surgical procedure in which incisions are made in both the left and right atrial walls which surround the pulmonary vein ostia and which leave a “maze-like” pathway between the sino-atrial node and the atrio-ventricular node. The incisions are sewn back together but result in a scar line which acts as a barrier to electrical conduction.
Although the “maze” procedure has its advantages, in practice it can be a complicated and a particularly risky procedure to perform since the surgeon is making numerous physical incisions in the heart tissue. Due in part to the risky nature of the maze procedure, alternative, catheter-based treatments have been advanced. Many of these catheter devices create the desired electrical block by way of ablation devices designed to burn lesions into the target tissue. Examples of these devices can be seen in U.S. patents: U.S. Pat. No. 6,254,599 to Lesh; U.S. Pat. No. 5,617,854 to Munsif; U.S. Pat. No. 4,898,591 to Jang et al.; U.S. Pat. No. 5,487,385 to Avitall; and U.S. Pat. No. 5,582,609 to Swanson, all incorporated herein by reference.
Although ablation catheter procedures remain less invasive than previous surgical methods like the “maze” procedure, they nevertheless retain a significant element of risk. For example, ablation procedures often utilize high power RF energy or ultrasonic energy, which may adequately create electrical block, but their inherent destructive nature allows for the possibility of unintended damage to the target tissue or nearby areas.
These techniques are used most often in the left or right atriums by creating electrical block either at discrete sites or along linear paths. Typically, the sites being targeted are referenced from landmarks in the chambers of the heart such as the ostium of the coronary sinus, the pulmonary veins, the tricuspid valve, the mitral valve, and the inferior and superior vena cava.
Currently, commonly used ablation devices are introduced percutaneously and advanced into the right atrium via the vena cava and possibly into the left atrium by a transeptal sheath. The ablation devices are then maneuvered inside the appropriate chamber of the heart by torquing the shaft of the catheter and deflecting the tip to bring the ablation tip in contact with the desired target site.
Positioning these ablation devices accurately is difficult as the atrium is a relatively large chamber, having a highly variable pulmonary vein anatomy which varies from patient to patient. Additionally, the atrium is constantly moving due to the beating of the heart and encounters large volumes of blood moving to and from the pulmonary veins. The blood flow causes difficulty because typical fluoroscopic techniques of injecting dye into the blood flow and allowing this to be carried by the blood to fill and illuminate the desired anatomy require large volume dye injections. Indeed, in most interventional applications, multiple dye injections are needed to periodically check the status of the procedure. This is typically not possible in the pulmonary veins due to the large volume of dye required for each injection and the fact that a patient can only tolerate a limited volume of dye without harming the kidneys.
One technique currently used to guide ablation catheters within this difficult environment involves a Lasso™ circular mapping catheter, manufactured by Biosense Webster which is a Johnson & Johnson company, that places radiopaque mapping electrodes around the perimeter of the ostium of the pulmonary veins. The Lasso™ circular mapping catheter is so named for its distal end, heat-set to curl into a ring or lasso shape. A Lasso™ catheter used for this procedure will typically have the radiopaque electrodes embedded within the lasso segment which allows the ring to be used as a physical and visual guide for an ablation catheter. Typically, the Lasso™ catheter is positioned into the atrium until it seats at the ostium of the pulmonary veins. The radiopaque electrodes act as an atrial ruler for guiding the ablation catheter to ablate around the ostium.
The Lasso™ catheter, however, is not an optimal solution for such ablation procedures since the Lasso™ catheter may be easily pushed into the pulmonary vein, causing the doctor to ablate inside the vein instead of around the ostium. Ablation within the pulmonary vein increases the risk of pulmonary vein stenosis and is therefore typically avoided.
In addition, as previously noted, the geometry of the pulmonary vein ostium is highly variable, often being more oval than round. Such variations can cause the Lasso™ catheter to be improperly positioned, further complicating ablation procedures.
In view of the above, it is apparent that there is a need for a positioning system which can more accurately guide interventions or delivery of devices to the target atrial site (e.g., the ostium of the pulmonary veins) minimizing the need for fluoroscopic dye injections.